A first conventional optical receiver board comprises an optical amplifier for amplifying an input signal light of a wavelength .lambda..sub.0, an optical bandpass filter having a transmission center wavelength .lambda..sub.0, for transmitting the signal light, a photodiode for converting the signal light to an electric signal, an equalizing amplifier for amplifying the electric signal, a timing sample circuit for sampling a timing signal from an amplified electric signal, and a discrimination circuit for waveform-shaping the amplified electric signal in accordance with the timing signal to provide an information electric signal.
In the first conventional optical receiver board, the input signal light is amplified in the optical amplifier, and the amplified signal light is transmitted through the optical bandpass filter, so that noise generated due to the amplified Spontaneous Emission (simplified "ASE" hereinafter) in the optical amplifier is removed except for the wavelength band of the signal light. The transmitted signal light is converted in the photodiode to the electric signal which is then amplified in the equalizing amplifier. The timing signal is sampled in the timing sample circuit to be supplied to the discrimination circuit in which the amplified electric signal is waveform-shaped in accordance with the timing signal to generate the information electric signal.
A second conventional optical receiver board comprises an optical fiber amplifier for amplifying an input signal light of wavelengths .lambda..sub.1 and .lambda..sub.2, an optical divider (splitter) for dividing (splitting) the input signal light amplified in the optical fiber amplifier into first and second signal lights, and first and second optical receiver units for generating first and second information electric signals from the first and second signal lights, wherein the first and second optical receiver units are the same in structure as the first conventional optical receiver board except that the first and second optical receiver units have no optical amplifier, because the optical fiber amplifier is provided at the front stage of the optical divider commonly to the first and second optical receiver units.
In the second conventional optical receiver board, the input signal light is amplified in the optical fiber amplifier, and the amplified signal light is divided in the optical divider to provide the first and second signal lights, respectively, to be supplied to the first an second optical receiver units. In the first optical receiver unit, the first information electric signal is obtained from the first signal light of the wavelength .lambda..sub.1 in the same operation as the first conventional optical receiver board. In the same manner, the second information electric signal is obtained from the second signal light of the wavelength .lambda..sub.2 in the second optical receiver unit. As the number of optical receiver unit is increased, the number of wavelengths to be multiplexed in a signal light can be increased.
In the first and second conventional optical receiver boards, the bandpass filter may be replaced by an optical wavelength-tuning filter module (often simplified "wavelength-tuning filter" hereinafter).
A conventional optical wavelength-tuning filter comprises input and output collimators and a multi-layered film interference filter provided between the input and output collimators, wherein that multi-layered film interference filter is inclined by an actuator, so that a signal light of a specified wavelength bandwidth is selected from a wavelength multiplexed signal light. Such an actuator comprises a ultrasonic motor composed of a rotor which is rotated relative to a stator in accordance with the piezo-electric effect as disclosed in the Japanese Patent Kokai No. 5-241083.
In the second optical receiver board, however, there are disadvantages in that an ideal receiving sensitivity is not obtained, the fabrication cost is high, the operation and maintenance of a wavelength-multiplexing communication system utilizing the second conventional receiver board are complicated, and the flexibility of the communication system is deteriorated. The detailed reasons will be explained prior to the disclosure of the invention.
In addition, the conventional wavelength-tuning filter has disadvantages in that the precision and dynamic characteristics of the actuator are not sufficient, because a rotating force is generated in the ultrasonic motor in accordance with frictional forces between piezo-electric ceramics and the stator, and the stator and the rotor, and that the manipulation is complicated, because its inclination angle must be adjusted to make a transmission center wavelength of a selected signal light constant, when if a collimated signal light supplied from an input collimator is changed in angle incident thereto due to the atmospheric change such as temperature, etc. At the same time, the conventional actuator for the optical wavelength-tuning filter module has disadvantages in that the size is large, a driving voltage is high, and the durability is low.
A further disadvantage is found in the conventional wavelength-tuning filter in that a polarization dependent loss ("PDL" hereinafter) inevitably fluctuates with an intensity of a received signal light. The detailed reasons will be explained prior to the disclosure of the invention.